The vanadates are among
my favorite group of minerals, and along with the phosphates and arsenates, are
usually grouped and studied together. In
these three groups, arsenic (As) or phosphorous (P) or vanadium (V) combine
with oxygen (O) to from the arsenate (AsO4), phosphate (PO4)
and vanadate (VO4) radicals.
Each of these radicals, with a negative charge of 3- then combines with a positive charged cation
metal(s), and often with water (H2O) or hydroxide (OH), to form a
wide variety of minerals. Since the
three radicals are approximately the same size they often substitute for one
another in a solid solution series. For
example, pyromorphite [lead phosphate [Pb5(PO4)3Cl]
is in solid solution with mimetite [lead arsenate Pb5(AsO4)3Cl]---the
negatively charged radicals change. The
latter mineral is usually a pale yellow to yellow-brown color while
pyromorphite is usually green to yellow-green in color; however, intermediate
stages in the solid solution series are known (from work with XRD or EDS or
other gizmos). Each of these radical
groups may also combine with a variety of metals (cations with a positive
charge) that often form solid solution series with each other. For example, erythrite [cobalt arsenate] is
in a complete solid solution series with annabergite [nickel arsenate] as the cobalt
cation substitutes for the nickel cation: Co3(AsO4)2-8(H20)
to Ni3(AsO4)2-8(H20). Therefore, it
is easy to understand the wide range, number and variety of arsenate, phosphate
and vanadate minerals when so many combinations of cations and radicals are
possible.
Many
arsenate—phosphate—vanadate minerals are bright in color, have easily observable
crystals and are widely available at mineral shows. Therefore, I am a sucker, actually a buyer,
whenever these minerals are located at shows (if the price is right)!
The arsenates and the
phosphates are well known minerals such as copper arsenates: olivenite and
clinoclase and cornwallite; and copper, zinc arsenate: austinite; cobalt arsenate:
erythrite; lead arsenate: mimetite; and nickel arsenate: annabergite. The phosphates include such minerals as
calcium phosphate: apatite group; lead phosphate: pyromorphite; lithium
phosphate: triphylite and amblygonite; copper aluminum phosphate: turquoise;
and the rare-earth phosphates: monazite and xenotime.
The vanadates are not
nearly as well know, or as common, as the previous groups but do include carnotite,
a uranium vanadate; mottramite and descloizite, copper-rich and zinc-rich
vanadates forming a solid solution series; and the best known of the group,
vanadinite, the red to orange lead mineral with beautiful and collectible
hexagonal crystals.
At the 2016 Tucson
Show I was rummaging through some minerals at Shannon and Sons toward closing
time and came upon a specimen of pottsite.
Normally I would have known little about this strange mineral except I
had been reading about minerals containing bismuth (Bi). Pottsite is a quite rare hydrated (H2O)
lead and bismuth vanadate [(Pb3Bi)Bi(VO4)4-H2O]
found in the oxide zones of tungsten-bearing rocks. MinDat noted that pottsite is the only natural
lead-bismuth vanadate known. The Pb/Bi
ratio varies from0.86 to 1.48. At the rock and mineral shows that I frequent pottsite
is not a common mineral for sale as the mineral has only been found in four localities
(MinDat): Cordoba, Argentina; Bavaria, Germany; and Nevada, USA (Churchill and
Lander counties). It seems as most of
the collected specimens come from the type locality, the Linka (AKA Garnetite)
Mine, Spencer Hot Springs District in Lander County. The major target at the Linka was tungsten with
slight recovery of copper and molybdenum.
Sherlock and others (1996) defined the Spencer Hot Springs District as a
“Tungsten Skarn” where scheelite-bearing [calcium tungstate], calc-silicate
rocks are formed at boundaries of hot magma bodies (a granodiorite at Linka)
and carbonate rocks. The hot fluids
dissolve some of the carbonate rocks (a process of metamorphism called
metasomatism) and deposit a wide variety of minerals dependent upon the composition
of the hydrothermal fluid. Evidently at
the Spencer Hots Springs District, tungsten was a major component of the fluids
along with secondary? lead, vanadium and bismuth. I remain uncertain as to the rareness of
combining lead and bismuth.
 |
| Macro photograph showing crust of yellow microscopic crystals of pottsite. Width of specimen ~1.5 cm. |
Pottsite occurs as a
bright yellow, almost druse, of microscopic (usually submillimeter) prismatic crystals
(Tetragonal), or as stubby prisms or bipyramids. They appear to be translucent to transparent and
are soft at ~3.5 (Mohs).
 |
| All of the above are photomicrographs showing submilimeter prismatic to stubby crystals of yellow pottsite. I am simply uncertain about the globular orange minerals. the chalky white mineral may be bismutite that lost the copper component? These were also the best enlargements that I could produce with my equipment. |
The mineralization
process concluding with the formation of pottsite is a complex sequence of
events. Williams (1988), in describing
this new mineral, pointed out: Pottsite is a product of oxidation that
followed these events [late metamorphism]. Junoite [copper lead bismuth sulfide] was first replaced by waxy green Bismutite
[bismuth carbonate] streaked with grey
cerussite [lead carbonate]; the green
bismutite then lost copper and became chalky white. Typically the bismutite was
then converted to a powdery orange (unknown) bismuth vanadate which, in turn
was replaced by clinobisvanite [bismuth vanadate]. Sparkling crusts of this mineral are commonplace in fractures
anywhere close to oxidized junoite. In a few spots pottsite has replaced the
unknown bismuth vanadate instead of clinobisvanite. It does not occur in
association with clinobisvanite.
I originally thought that
perhaps some orange material in the small sample was clinobisvanite (BiVO4). However, I could not locate any fluorescence in
the specimen---any indicator of clinobisvanite.
I also remain somewhat confused (not all that difficult) with the
statement by Williams (1988): It [pottsite] does not occur in association with
clinobisvanite. My confusion relates
to photos on MinDat showing specimens with both minerals present. Perhaps new studies since 1988 have shown
both minerals may be found together? One
of life’s persistent questions!
Speaking of those
questions: Life’s most persistent and urgent question is “What are you doing for
others?”
Martin Luther King,
Jr.
REFERENCE CITED
Sherlock, M.G., D.P.
Cox, and D.F. Huber, 1996, Known
mineral deposits and occurrences in Nevada: in
Chapter 10 from Nevada Bureau of Mines and Geology Open-File Report 96-2: An
analysis of Nevada's metal-bearing mineral resources): www.Nnsa.energy.gov/sites/dsfault/files/nnsa/imlinefiles
Williams, S.A., 1988,
Pottsite, a new vanadate from Lander County, Nevada: Mineralogical Magazine, v.
52.
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